In order to restore the function of a loose artificial hip joint, various major and minor problems have to be surmounted. The major problems are the anchorage problems related to achieving stable fixation despite often large defects remaining in the bony support after the joint components have been removed. Minor problems involve filling in the defects with bone from tissue banks; this is accomplished using xe2x80x9cmorcellized bonexe2x80x9d plastics of the appropriate size. (Lamerigts, N. M. P., 1998. Proefschrift an der katholischen Universitt Niymegen.) Once the bony support structure has been reinforced with bone from tissue banks, the corresponding joint replacement components can be cemented in.
In order to use such a procedure, the bony structures must be sufficiently stable to achieve a stable overall anchorage. However, these bone structures often are no longer present, and as a result, very special demands are placed on the implant. Therefore, there is a genuine need for systems that can be adapted to the given situation that is when large defects are present, and that take various biomechanical fixation principles into account. With this background as a foundation, a novel approach to the problem of revision operations (i.e., replacement of the femur component of a prosthesis) was unexpectedly discovered.
The extent of the defects in the bony femur bed after the removal of a loose, previously implanted prosthesis may vary. This has led to attempts to classify bone defects, for example in the DGOT (Bettin, D., Katthagen, B. D., (1997), Die DGOT-Klassifkation von Knochendefekten bei Huft-Totalendoprothese-Rev-isionsoperationen [The DGOT Classification of Bone Defects in Total Hip Endoprosthesis Revision Operations], Z. Orthop. 135). In some cases, the bone damage is considerable. Treatment of the loose prosthesis or implant components involves complete removal of the components and, if present, the bone cement that was previously used, as well as all of the connective tissue surrounding the previous implant, that connects the implant to the bone. Not until this has been done can one realistically assess the extent of bone loss. Often, the only way to anchor a new implant component is to reach beyond all defects and anchor the component deep in the portion of the femur diaphysis (the shaft of the long bone) that is still healthy, frequently without the use of cement.
Another method is to reconstruct the bone with morcellized bone from tissue banks and use cement to reattach such a component. This is described in detail in Lamerigts, N. M., (1998), The Incorporative Process of Morcellized Bone Graft. Proefschrift University Nijmegen (Catholic University). In both cases, proximal anchoring, that is, near the upper end of the bone is usually not stable. The implants in the femur are usually very long and heavy, and much poorer results are obtained than in primary operations.
Tests and simple experiments on cadaver bones unexpectedly revealed very efficient ways to anchor and fix femur components in defective bone support structures, even components having short shafts.
A prosthesis anchorage system comprises a modular replacement insert for the femur for repairing artificial hip joints. The anchorage system provides a femur stem that is segmented and will insert into the femur canal, and can be elongated to a length so that portions of the femur stem will be aligned in the canal with bone that provides a solid holding action for the stem when replacing a previously installed stem that has become loosened. The base modular section includes an axial or central cylinder that inserts into adjacent stem sections and which serves to hold the stem sections in alignment. At the proximal end of the femur, a shoulder stem section is used. The shoulder stem section has a surface that will permit attaching a mating shoulder on a neck and head prosthesis securely with tension carrying members.
The length of the femur stem inserted into the femur canal can be adjusted to accommodate a wide variety of conditions when a hip joint is to be replaced.